The invention relates to an interferometer including a thin, absorbing attenuator in the path of a test beam to balance optical intensities of the test beam and an interfering reference beam, thereby producing distinct interference fringes and reducing effects of spurious reflections.
Interferometers produce interference fringes representing contours of a test surface, as a result of interference between a light beam reflected from a known reference surface and a light beam reflected from a test surface. It is well known that contrast of the interference fringes is inadequate if the intensity of the beam reflected from the test surface does match the intensity of the beam reflected from the reference surface. For a Twyman-Green interferometer, it frequently is necessary for a user to maintain an inventory of expensive reference surfaces of various reflectivities to match the reflectivities of a broad range of test surfaces that may need to be measured. For computerized phase measurement systems, it is necessary to have cosinusoidal fringes. In a Fizeau interferometer, if the test surface is of higher reflectivity than the reference surface, it is necessary to attenuate the test beam to maintain the cosinusoidal fringes.
U.S. Pat. 3,998,553 (Hunter et al.) discloses a spherical wavefront Fizeau interferometer that utilizes a commercially available metal coated pellicle located between the reference surface and the test surface to balance the intensity of a test beam with that of a reference beam before allowing interference between them. Unfortunately, the metal coated surface of the pellical may produce spurious reflections that may interfere with the computerized measurement of the interference pattern. More specifically, the spurious reflections may produce indeterminate gaps or "holes" in the data representing the shape of the computer-calculated wavefront returned from the test surface. FIG. 1 illustrates a typical fringe pattern having fringes such as 9A, 9B, and 9C to be measured by a computerized interferometer. Reference numeral 11 designates a large erroneous "spike" in the fringe intensity profile data, causing inaccurate phase measurements. The coated pellicle can be tilted to avoid effects of the stray reflections, but such tilting is undesirable because for large spherical test surfaces there may not be enough available space in the interferometer setup to allow the needed tilting of the pellicle. The manufacturing yield of commercially available coated pellicles is lower than desirable and the number of steps required for their manufacture is higher than desirable, leading to unnecessarily high cost.
Thus, there is an unmet need for an improved interferometer that avoids the problems caused by prior reflective coated pellicles utilized to balance the intensity of a test beam with a reference beam.